Method and device for energy efficient signal transmission in massive multi-antenna wireless communication system

ABSTRACT

A method for selecting a Multiple-Input Multiple-Output (MIMO) mode based on energy efficiency and selecting an antenna subset to be used in a communication, in a wireless communication system using a multi-user massive multi-antenna is provided. The method selects an antenna subset capable of minimizing transmission power without using all antennas, based on power consumed by an Radio Frequency (RF) circuit as well as the transmission power. The method further includes selecting a mobile station to which the signal is to be transmitted, selecting a multi-antenna technique based on power consumption of all antennas, selecting an antenna subset to transmit the signal to the mobile station among all the antennas, and transmitting the signal to the mobile station by using the antenna subset.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit under 35 U.S.C. §119(e) of a U.S.provisional patent application filed on Jun. 21, 2013 in the U.S. Patentand Trademark Office and assigned Ser. No. 61/837,898, and under 35U.S.C. §119(a) of a Korean patent application filed on Oct. 8, 2013 inthe Korean Intellectual Property Office and assigned Serial number10-2013-0120132, the entire disclosure of each of which is herebyincorporated by reference.

JOINT RESEARCH AGREEMENT

The present disclosure was made by or on behalf of the below listedparties to a joint research agreement. The joint research agreement wasin effect on or before the date the present disclosure was made and thepresent disclosure was made as a result of activities undertaken withinthe scope of the joint research agreement. The parties to the jointresearch agreement are 1) SAMSUNG ELECTRONICS CO., LTD. and 2) SNU R &DB FOUNDATION.

TECHNICAL FIELD

The present disclosure relates to an apparatus and a method forincreasing energy efficiency of a base station in a wirelesscommunication system. More particularly, the present disclosure relatesto a signal transmission technique for selecting an antenna mode toincrease energy efficiency of a base station, and selecting an antennasubset to transmit signals to a mobile station among all antennas, in awireless communication system using a multi-user massive multi-antenna.

BACKGROUND

Wireless data traffic has explosively increased due to invigoration of awireless multimedia service and a Social Networking Service (SNS)according to the spread of smart phone use and the expansion of wirelessdemands, such as Machine To Machine communication. Accordingly, a bigdata environment which is not easy to process through data transmissionmethods of the related art is imminent.

With the advent of the big data environment, a Massive Multiple-InputMultiple-Output (M-MIMO) system is being considered for efficientlymanaging wireless resources. Further, the M-MIMO system is beingspotlighted as an energy efficient green communication technology.

Studies on the wireless communication system of the related art havebeen focused on the increase in a channel capacity, such as installationof more base stations or securing of a frequency band. For example,technologies of the related art including Orthogonal Frequency DivisionMultiple Access (OFDMA), a multi-antenna system, and a relaytransmission system attempt to provide a high channel capacity.

However, high energy consumption is inevitably required to provide thehigh channel capacity. Considering that, the method for raising thechannel capacity is not efficient for devices with restricted totalenergy or networks focused on energy efficiency.

Meanwhile, with the advent of the M-MIMO system and the distributedantenna system, it is easier, in recent years, to satisfy transmissionrequirements of a mobile station based on abundant resources.Accordingly, a wireless communication system is required which ismaximally energy efficient and satisfies the transmission requirementsof the mobile station.

In the case of transmission techniques of the related art consideringenergy efficiency, a plurality of antennas are used so that transmissionpower but not power consumption is considered. However, due to theadvent of the M-MIMO system, more power should be consumed to operate somany antennas.

Therefore, a need exists for a signal transmission method for satisfyinga user's transmission requirements and improving power efficiency of abase station based on power consumption in a Radio Frequency (RF)circuit as well as transmission power.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present disclosure.

SUMMARY

Aspects of the present disclosure are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentdisclosure is to provide a signal transmission method for satisfying auser's transmission requirements and improving power efficiency of abase station based on power consumption in a Radio Frequency (RF)circuit as well as transmission power.

In accordance with an aspect of the present disclosure, a method fortransmitting a signal by a base station in wireless communication usinga massive multi-antenna technique is provided. The method includesselecting a mobile station to which the signal is to be transmitted,selecting a multi-antenna technique based on power consumption of allantennas, selecting an antenna subset to transmit the signal to themobile station among all the antennas, and transmitting the signal tothe mobile station by using the antenna subset.

The selecting of the multi-antenna technique may include calculatingpower of all the antennas for each of multi-antenna techniques andselecting a multi-antenna technique requiring less power.

The selecting of the antenna subset may include selecting the antennasubset based on the selected multi-antenna technique and channel gainsfor the respective antennas or a correlation between the antennas.

In accordance with another aspect of the present disclosure, a basestation for transmitting a signal to a mobile station in wirelesscommunication using a massive multi-antenna technique is provided. Thebase station includes a transmission/reception unit configured totransmit/receive the signal to/from the mobile station, and a controllerconfigured to select a mobile station to which a signal is to betransmitted, to select a multi-antenna technique based on powerconsumption of all antennas, to select an antenna subset to transmit thesignal to the mobile station among all the antennas, and to control thesignal to be transmitted to the mobile station by using the antennasubset.

The controller may calculate power of all the antennas for each ofmulti-antenna techniques and select a multi-antenna technique requiringless power.

The controller may select the antenna subset based on a selectedmulti-antenna technique, and channel gains for the respective antennasor correlations between the antennas.

As described above, the present disclosure can provide an energyefficient user scheduling method, and has an effect of obtaining anadditional gain by selecting an efficient multi-antenna technique.

In addition, according to the present disclosure, power efficiency ofthe base station can be improved, and at the same time, the number ofadditionally used antennas is decreased so that complexity can also bedecreased.

Other aspects, advantages, and salient features of the disclosure willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses various embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainembodiments of the present disclosure will be more apparent from thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a network configuration diagram illustrating a relationshipbetween a base station and mobile stations in an energy efficient signaltransmission method according to an embodiment of the presentdisclosure;

FIG. 2A is a flowchart illustrating an energy efficient signaltransmission method according to an embodiment of the presentdisclosure;

FIG. 2B is a flowchart illustrating an energy efficient signaltransmission method according to an embodiment of the presentdisclosure;

FIG. 3 is a flowchart illustrating a process of selecting a user in anenergy efficient signal transmission method according to an embodimentof the present disclosure; and

FIG. 4 is a flowchart illustrating a process of selecting an antennasubset according to antenna techniques in an energy efficient signaltransmission method according to an embodiment of the presentdisclosure.

Throughout the drawings, it should be noted that like reference numbersare used to depict the same or similar elements, features, andstructures.

DETAILED DESCRIPTION

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of variousembodiments of the present disclosure as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the various embodiments describedherein can be made without departing from the scope and spirit of thepresent disclosure. In addition, descriptions of well-known functionsand constructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of the presentdisclosure. Accordingly, it should be apparent to those skilled in theart that the following description of various embodiments of the presentdisclosure is provided for illustration purpose only and not for thepurpose of limiting the present disclosure as defined by the appendedclaims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to skill in theart, may occur in amounts that do not preclude the effect thecharacteristic was intended to provide.

FIG. 1 is a network configuration diagram illustrating a relationshipbetween a base station and mobile stations in an energy efficient signaltransmission method according to an embodiment of the presentdisclosure.

Referring to FIG. 1, it is assumed that a base station 101 uses N_(T)antennas 102 and there are a total of K mobile stations to which thebase station 101 has to transmit a signal. When the base station 101selects, through a mobile station selector 103, K_(m) mobile stations towhich signals are simultaneously transmitted, the base station 101 formssymbol signals to transmit to the K_(m) mobile stations by using asignal generator 104, and generates signals to transmit through abeam-former 105, a reception signal of a mobile station (k) 106receiving a service from the base station 101 may be represented asfollows.

$\begin{matrix}{y_{k} = {{\sqrt{\alpha_{k}P_{k}}h_{k}w_{k}s_{k}} + {\overset{K_{m}}{\sum\limits_{{l = 1},{l \neq k}}}{\sqrt{\alpha_{k}P_{k}}h_{m}w_{l}s_{l}}} + n_{k}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Here, ^(α) ^(k) denotes a path loss from the base sttation 101 to themobile station (k) 106, P denotes transmission power transmitted to themobile station (k) 106 by the base station 101, ^(h) ^(k) denotes a^((1×N) ^(T) ⁾ channel vector from the base station 101 to the mobilestation (k) 106, ^(w) ^(k) and ^(s) ^(k) denote a ^((N) ^(T) ^(×1)) beamweight vector and a transmission signal, respectively, transmitted bythe base station 101, and ^(n) ^(k) denotes an adaptive white Gaussiannoise having an average of 0 and a variable of ^(σ) ^(n,k) ² . At thistime, total power consumption of the base station may be represented byEquation 2.

$\begin{matrix}{P = {{{\sum\limits_{k = 1}^{K}P_{k}} + {P_{RF}N_{T}}} = {P_{T} + {P_{RF}N_{T}}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Here, ^(P) ^(RF) denotes power consumption of a Radio Frequency (RF)circuit required by one antenna when the base station transmits asignal.

FIG. 2A is a flowchart illustrating an energy efficient signaltransmission method according to an embodiment of the presentdisclosure.

Referring to FIG. 2A, in operation 210, a base station receives, from anupper layer, transmission requirements and transmission signals of kmobile stations requiring data transmission, and calculates thetransmission requirements for the respective mobile stations.

In operation 220, the base station may calculate transmission power ^(P)^(o,k) required for data transmission through a single antenna based onthe transmission requirements and channel environments of the mobilestations. Thereafter, the base station may select the mobile stationbased on the calculated transmission power.

Thereafter, in operation 230, the base station determines amulti-antenna technique for the selected mobile station. For example,the base station may select a multi-antenna technique consuming lesstransmission power between Maximal Ratio Transmission (MRT) andZero-Forcing Beam-Forming (ZFBF).

FIG. 2B is a flowchart illustrating an energy efficient signaltransmission method according to an embodiment of the presentdisclosure.

Referring to FIG. 2B, similar to FIG. 2A, a base station may receive,from an upper layer, transmission requirements and transmission signalsof k mobile stations requiring data transmission and may calculate thetransmission requirements for the respective mobile stations, inoperation 210, and may calculate transmission power ^(P) ^(o,k) requiredfor data transmission through a single antenna based on the transmissionrequirements and channel environments of the mobile stations and then,may select the mobile station based on the calculated transmissionpower, in operation 220.

Thereafter, in operation 240, the base station may calculate the number^(N) ^(T,sel) of antennas by which the transmission power is minimizedwhen transmission is made through the selected multi-antenna technique,and may select an antenna subset capable of maximizing transmissionefficiency among a total of ^(N) ^(T) antennas.

Meanwhile, although FIGS. 2A and 2B have been differentiated as separatedrawings in the present disclosure, the present disclosure is notlimited thereto. Namely, the signal transmission methods of FIGS. 2A and2B may be understood as a single process. In this case, operation 240 ofFIG. 2B may progress after operation 230 of FIG. 2A.

Two methods of selecting an antenna may be largely exemplified. A firstmethod is a method for selecting an antenna through a comparison ofchannel gains for respective antennas, and a second method is a methodfor selecting an antenna based on correlations between antennas.

Such a method for selecting the antenna may vary depending oncharacteristics of the determined Multiple-Input Multiple-Output (MIMO)mode. For example, in a case of selecting an antenna while the MIMO modeis determined as MRT, a performance is considerably enhanced with anincreasing channel gain, so that it is appropriate to select an antennahaving a large channel gain through a comparison of channel gains forrespective antennas.

As another example, in a case of selecting an antenna while the MIMOmode is determined as ZFBF, a performance is enhanced with an increasingrank of a channel matrix and the rank is increased with decreasingcorrelations between antennas, so that it is appropriate to select anantenna having the lowest correlation between the antennas.

In a case of Eigen Beam-Forming (EBF) as another MIMO technique, aperformance gain is large with an increasing channel correlation andthus, a determined number of contiguous antennas may be selected suchthat the highest correlation between antennas may be obtained.

FIG. 3 is a flowchart illustrating a process of selecting a user in anenergy efficient signal transmission method according to an embodimentof the present disclosure.

Referring to FIG. 3, in operation 301, a base station receivestransmission requirements and transmission signals of all mobilestations from an upper layer. Here, when frequency efficiency requiredby a mobile station k is ^(C) ^(k) , transmission power required whentransmission is made to the mobile station k may be calculated accordingto the number of multiplexings and the number of selected antennas. Asan example, when the base station determines a multi-antenna techniqueas ZFBF, transmission power required when transmission is made to themobile station k may be represented by Equation 3.

$\begin{matrix}\begin{matrix}{P_{k} = \frac{N_{0}\left( {2^{C_{k}} - 1} \right)}{\alpha_{k}\left( {N_{T,{sel}} - K_{m} + 1} \right)}} \\{= \frac{P_{o,k}}{\left( {N_{T,{sel}} - K_{m} + 1} \right)}}\end{matrix} & {{Equation}\mspace{14mu} 3}\end{matrix}$

Thus, total power consumption may be represented by Equation 4 when thebase station transmits signals through ^(N) ^(T,sel) antennas.

$\begin{matrix}\begin{matrix}{P = {{\sum\limits_{k = 1}^{K_{m}}\frac{\sigma_{n,k}^{2}\left( {2^{C_{k}} - 1} \right)}{\alpha_{k}\left( {N_{T,{sel}} - K_{m} + 1} \right)}} + {P_{RF}N_{T,{sel}}} + {P_{idle}\left( {N_{T} - N_{T,{sel}}} \right)}}} \\{= {{\frac{1}{\left( {N_{T,{sel}} - K_{m} + 1} \right)}{\sum\limits_{k = 1}^{K_{m}}P_{o,k}}} + {P_{RF}N_{T,{sel}}} + {P_{idle}\left( {N_{T} - N_{T,{sel}}} \right)}}}\end{matrix} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Since ^(K) ^(m) ^(≦N) ^(T,sel) , Equation 4 is a function that increasesaccording to the number of multiplexings ^(K) ^(m) . Accordingly, it canbe seen that power consumption decreases as the number of multiplexingsdecreases, and it can be identified that a mobile station should beselected based on transmission power ^(P) ^(o,k) required when the basestation transmits a signal to the mobile station k by using a singleantenna.

Thus, in operation 302, the number of multiplexings which the basestation should transmit through one resource block is determined. As anexample, when the base station has ^(N) ^(RB) resource blocks, thenumber of multiplexings for minimizing the number of multiplexings inall the resource blocks may be represented by Equation 5.

$\begin{matrix}{K_{m} = \left\lceil \frac{K}{N_{RB}} \right\rceil} & {{Equation}\mspace{20mu} 5}\end{matrix}$

In operation 303, the transmission power ^(P) ^(o,k) required when thebase station transmits a signal to the mobile station k by using asingle antenna is calculated and may be represented by Equation 6.

$\begin{matrix}{P_{o,k} = \frac{\sigma_{n,k}^{2}\left( {2^{C_{k}} - 1} \right)}{\alpha_{k}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

In operation 304, mobile stations are selected such that a sum of ^(P)^(o,k) for ^(K) ^(m) mobile stations to which transmission issimultaneously made through one resource block is made identical foreach of the resource blocks.

FIG. 4 is a flowchart illustrating a process of selecting an antennasubset according to antenna techniques in an energy efficient signaltransmission method according to an embodiment of the presentdisclosure.

Referring to FIG. 4, it illustrates a process of determining amulti-antenna technique (MRT, ZFBF) and selecting an antenna in awireless communication system to which energy efficient terminalselection and antenna selection techniques proposed by the presentdisclosure are applied, and expansion to another multi-antenna techniquemay be easily described. In operations 401 and 402, the number ofantennas required for respective multi-antenna techniques is calculated,and the number of antennas required when transmission is made throughMRT may be represented by Equation 7.

$\begin{matrix}{N_{T,{sel}}^{({MRT})} = \left\lceil \sqrt{\frac{\sum\limits_{k = 1}^{K_{m}}\left( {P_{o,k} + P_{i,k}} \right)}{P_{cir} - P_{idle}}} \right\rceil} & {{Equation}\mspace{14mu} 7}\end{matrix}$

Here, ^(P) ^(i,k) denotes transmission power additionally required byinterferences between multiple beams and may be represented by Equation8.

$\begin{matrix}{P_{i,k} = {\left( {2^{C_{k}} - 1} \right){\sum\limits_{{l = 1},{l \neq k}}^{K_{m}}P_{l}}}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

The number of antennas required when transmission is made through ZFBFmay be represented by Equation 9.

$\begin{matrix}{N_{T,{sel}}^{({ZFBF})} = \left\lceil {\sqrt{\frac{\sum\limits_{k = 1}^{K_{m}}P_{o,k}}{P_{cir} - P_{idle}}} + K_{m} - 1} \right\rceil} & {{Equation}\mspace{14mu} 9}\end{matrix}$

In operation 403, total power consumption required by the base stationwhen signals are transmitted through MRT is calculated, and in operation404, total power consumption required by the base station when signalsare transmitted through ZFBF is calculated. In the case of transmissionthrough MRT, the total power consumption may be represented by Equation10.

$\begin{matrix}\begin{matrix}{P^{({MRT})} = {{\sum\limits_{k = 1}^{K_{m}}{\frac{\left( {{\sum\limits_{{l = 1},{l \neq k}}^{K_{m}}{\alpha_{k}P_{l}}} + \sigma_{n,k}^{2}} \right)\left( {2^{C_{k}} - 1} \right)}{\alpha_{k}N_{T,{sel}}^{({MRT})}}P_{RF}N_{T,{sel}}^{({MRT})}}} +}} \\{{P_{idle}\left( {N_{T} - N_{T,{sel}}^{({MRT})}} \right)}} \\{= {{\frac{1}{N_{T,{sel}}^{({MRT})}}{\sum\limits_{k = 1}^{K_{m}}\left( {P_{i,k} + P_{o,k}} \right)}} + {P_{RF}N_{T,{sel}}^{({MRT})}} +}} \\{{P_{idle}\left( {N_{T} - N_{T,{sel}}^{({MRT})}} \right)}}\end{matrix} & {{Equation}\mspace{14mu} 10}\end{matrix}$

In the case of transmission through ZFBF, the total power consumptionmay be represented by Equation 11.

$\begin{matrix}{P^{({ZFBF})} = {{\frac{1}{\left( {N_{T,{sel}}^{({ZFBF})} - K_{m} + 1} \right)}{\sum\limits_{k = 1}^{K_{m}}P_{o,k}}} + {P_{RF}N_{T,{sel}}^{({ZFBF})}} + {P_{idle}\left( {N_{T} - N_{T,{sel}}^{({ZFBF})}} \right)}}} & {{Equation}\mspace{14mu} 11}\end{matrix}$

In operation 405, the multi-antenna technique requiring less power isdetermined through a comparison of the required power for respectivemulti-antenna techniques which has been calculated in operations 403 and404. In operation 406, antennas are selected based on the determinedmulti-antenna technique and the number of antennas. As an example, whenMRT is selected among the multi-antenna techniques, antennas areselected in a descending order of a channel gain such that abeam-forming gain of MRT can be maximized, and as another example, whenZFBF is selected among the multi-antenna techniques, antennas areselected such that a rank of multiple mobile station channels ismaximized.

Thereafter, the base station may select antennas based on the determinedMIMO mode and the number of antennas according to the MIMO mode, amongantennas retained by the base station.

Two methods of selecting the antennas may be largely exemplified. Afirst method is a method for selecting antennas through a comparison ofchannel gains for respective antennas, and a second method is a methodfor selecting antennas based on correlations between antennas.

Such a method for selecting the antennas may vary depending oncharacteristics of the determined MIMO mode. For example, in a case ofselecting the antennas while the MIMO mode is determined as MRT, aperformance is considerably enhanced with an increasing channel gain andthus, it is appropriate to select antennas having a large channel gainthrough a comparison of channel gains for respective antennas.

As another example, in a case of selecting the antennas while the MIMOmode is determined as ZFBF, a performance is enhanced with an increasingrank of a channel matrix and the rank increases with decreasingcorrelations between antennas, so that it is appropriate to selectantennas having the lowest correlation between the antennas.

In a case of EBF as another MIMO technique, a performance gain is largewith an increasing channel correlation and thus, a determined number ofcontiguous antennas may be selected such that the highest correlationbetween antennas may be obtained.

While the present disclosure has been shown and described with referenceto various embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and spirit of the present disclosureas defined by the appended claims and their equivalents.

What is claimed is:
 1. A method for transmitting a signal by a basestation in wireless communication using a massive multi-antennatechnique, the method comprising: selecting a mobile station to whichthe signal is to be transmitted; selecting an antenna subset to transmitthe signal to the mobile station among all antennas; and transmittingthe signal to the mobile station by using the antenna subset.
 2. Themethod of claim 1, wherein the selecting of the antenna subsetcomprises: selecting the antenna subset based on a selectedmulti-antenna technique, and channel gains for the respective antennasor correlations between the antennas.
 3. The method of claim 2, whereinthe selecting of the antenna subset further comprises: selectingantennas having a large channel gain as the antenna subset when themulti-antenna technique is determined as Maximal Ratio Transmission(MRT).
 4. The method of claim 2, wherein the selecting of the antennasubset further comprises: selecting antennas having the lowestcorrelation between the antennas as the antenna subset when themulti-antenna technique is determined as Zero-Forcing Beam-Forming(ZFBF).
 5. A method for transmitting a signal by a base station inwireless communication using a massive multi-antenna technique, themethod comprising: selecting a mobile station to which the signal is tobe transmitted; selecting a multi-antenna technique based on powerconsumption of all antennas; and transmitting the signal to the mobilestation by using an antenna subset.
 6. The method of claim 5, whereinthe selecting of the multi-antenna technique comprises: calculatingpower of all the antennas for each of multi-antenna techniques andselecting a multi-antenna technique requiring less power.
 7. A basestation for transmitting a signal to a mobile station in wirelesscommunication using a massive multi-antenna technique, the base stationcomprising: a transmission/reception unit configured to transmit/receivethe signal to/from the mobile station; and a controller configured toselect a mobile station to which a signal is to be transmitted, toselect an antenna subset to transmit the signal to the mobile stationamong all antennas, and to control the signal to be transmitted to themobile station by using the antenna subset.
 8. The base station of claim7, wherein the controller is further configured to select the antennasubset based on a selected multi-antenna technique, and channel gainsfor the respective antennas or correlations between the antennas.
 9. Thebase station of claim 8, wherein the controller is further configured toselect antennas having a large channel gain as the antenna subset whenthe multi-antenna technique is determined as Maximal Ratio Transmission(MRT).
 10. The base station of claim 8, wherein the base station isfurther configured to select antennas having the lowest correlationbetween the antennas as the antenna subset when the multi-antennatechnique is determined as Zero-Forcing Beam-Forming (ZFBF).
 11. A basestation for transmitting a signal to a mobile station in wirelesscommunication using a massive multi-antenna technique, the base stationcomprising: a transmission/reception unit configured to transmit/receivethe signal to/from the mobile station; and a controller configured toselect a mobile station to which a signal is to be transmitted, toselect a multi-antenna technique based on power consumption of allantennas, and to control the signal to be transmitted to the mobilestation by using an antenna subset.
 12. The base station of claim 11,wherein the controller is further configured to calculate power of allthe antennas for each of multi-antenna techniques and to select amulti-antenna technique requiring less power.